ON THE VERTEBRATE SKELETON. 



197 



to prevail, as in the case of the alisphenoid, in the determination of its special 

 homology. 



The supraoccipital, by virtue of its internal position and lodgment of part 

 of the labyrinth, has equal claims to the name of ' rocher,' according to the 

 Cuvierian characters of that bone, and Geoffroy St. Hilaire did not make a 

 less arbitrary choice in singling out this element as 'le seul rupeal*,' than 

 Cuvier did in choosing the alisphenoid, or, as any other anatomist would do 

 in preferring any other element of a cranial vertebra in the crocodile to 

 represent the ossified ear-capsule of the fish or mammal, because portions of 

 that ossified capsule are protected by, or have coalesced with, such vertebral 

 elements. Had Cuvier looked beyond the special homology of the bones of 

 the head of the crocodile, and permitted himself to appreciate their higher and 

 more general relations, he could scarcely have failed to perceive the corre- 

 spondence of his so-called 'rocher' in batrachians, ophidians, chelonians and 

 saurians, to the bone which he so well recognizes as ' the great wing of the 

 sphenoid' in the perch and cod-fish. 



The Mastoid. — In the human embryo of the fifth month a centre of ossi- 

 fication is established on the outer surface of the mass of cartilage occu- 

 pying the interspace between the basioccipital (fig. 11, 1) and exoccipital 



(2) below, the tympanic (2s) and squamosal (27) in front, the supraoccipital 



(3) behind, and the parietal (7) above: this mass of cartilage incloses the 

 membranous labyrinth, about which a light osseous crust has begun to be 

 formed ; and, from the centre (s) established near the outer border of the 

 posterior semicircular canal, ossification radiates to complete that part of the 

 cranial parietes, which, in the adult skull, is impressed on its inner surface by 

 the great venous channel called ' fossa sigmoidea,' and developes from its 

 outer surface the ' processus mastoi- 

 deus.' The primitive independence 

 of the base of this process, which 

 Kerkringius so clearly and accurately 

 delineates in his tab. xxxv.Jiff. iii. as 

 the posterior of his ' tria petrosi ossis 

 distincta ossiculaf ,' is a fact of much 

 more significance than its brief and 

 transitory manifestation would lead 

 the anthropotomist to divine. The 

 coalescence of the primitively distinct 

 mastoid with the ossifying capsule of 

 the labyrinth is very speedy, being 

 usually complete before the foetus has 

 passed its fifth month, and a com- 

 posite ' petro-mastoid' bone is thus 

 formed, which, retaining its indivi- 

 duality in monotremes, marsupials, 

 ruminants and many rodents, pro- 

 ceeds to coalesce with the additional 

 elements of the ' temporal ' bone in man, and with other surrounding cranial 

 bones in birds. In the cold-blooded vertebrata, the mastoid retains, with a few 

 exceptions, its primary embryonic distinctness, as an independent element of 

 the skull. In tracing the modifications of this element downwards from man, 

 we find the external process from which its anthropotomical name originated, 



Fig. 11. 



Skull of the human embryo ; fifth month. 

 Natural size. 



* Annales des Sciences Naturelles, torn. iii. 1824, p. 271, pi. 16. 

 f Spicilegium Anatomicum, 4to. 1670, Osteogenia Fcetuum, p. 269. 



